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The living cell is traditionally considered the “basic
building block” of living things. Plants and animals
that are familiar to most are all made up of millions
and millions of single cells that perform certain duties
to keep the animal or plant alive. Some plants and animals,
the amoeba, for example, are only as big as a single cell,
although they are large cells compared to many others.
One structure that is found in most cells is the outside
covering, or the “cell membrane.” The cell
membrane basically acts like the plastic skin of a water
balloon by keeping the inside parts of the cell inside
the cell, and also acts as a barrier to keep unwanted
things out of the cell.
However, the cell membranes of living cells are actually
not as solid as the skin of a water balloon. Cell membranes
are “semi-permeable” membranes, which means
that they actually have lots of tiny holes in them and
will allow some molecules to flow through them. Water
is one of those molecules. Because the water molecule
is pretty small with only 2 hydrogen atoms and 1 oxygen
atom, it can pass easily through the membranes of most
cells. But, when the water on one side of the cell membrane
has more salt, or sugar, or other materials dissolved
into it than the water on the other side, an interesting
thing will happen. The water that holds more sugar,
or salt, etc, actually has fewer water molecules per
unit of volume than the side with less salt, or sugar.
In other words, the side with less salt or sugar has
more water molecules bouncing off of it than the side
with the salt or sugar. This results in more “osmotic
pressure” on the less concentrated side, and water
molecules begin to travel through the membrane and into
the salty, or sugary water. This will continue until
the concentration of salt, or sugar, or whatever, becomes
equal on both sides of the cell membrane (unless, as
some specialized cells do, the cell has a mechanism
for moving these molecules in order to maintain different
concentrations on each side).
In the case of plant cells, like the cells in the skin
of an onion, there is another structure just outside
of the membrane. This structure is called the cell wall.
The cell wall is mostly made up of a tough material
called cellulose, which gives the cells a stronger,
more rigid structure than cells of animals. When plant
cells, such as those in an onion skin are put into salty
water, an interesting thing can be observed through
any of the three objectives of the Blister microscope.
As the water on the inside of the cells fights to get
out and equalize the concentration of salt on both sides
of the membrane, the cell shrinks, and so does the membrane
(like letting water out of a water balloon). However,
the cell walls, which are not totally attached to the
membrane, remain the same size as they were. When we
look at cells that have been exposed to salt water for
even only a few minutes, we can easily see the outline
of some of the shrunken cells and their membranes within
the cell walls.
Cells that are exposed to high concentrations of salt,
sugar, and many other substances can die as a result
of giving up too much of their water. They literally
die of thirst. This phenomenon has resulted in the use
of sugar and salt as food preservatives. Jellies, jams,
and many processed meat products rely on sugar or salt
to keep bacteria from spoiling the food so it can be
safely eaten for a long period of time. While none of
us can literally learn through osmosis (as some people
like to say about other people who learn things very
quickly), people have certainly learned to protect their
food sources because of osmosis.
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